1. Field of the Invention
The present invention relates to a device for measuring of expansion and contraction in general and particularly monitoring for the respiration of patients by moving coils producing a mutual induction for modulating a carrier signal.
2. Description of the Related Art
Many devices measure the respiration of patients. In one method a mask placed over the patient's mouth and nose measures flow rates of inhaled and exhaled breath. This is uncomfortable for the patient and requires a lot of cumbersome equipment.
Other methods of monitoring breathing include the measurement of thoracic impedance with the use of electrodes placed on the patient. However there are many electrical signals in the body from the heart and other muscles, which are picked up by such electrodes and interfere with the monitoring of the thoracic impedance.
Another method of monitoring a patient's breathing is by measuring the patient's torso to detect expansion and contraction, which is directly proportional to the volume and rate of inhaling and exhaling. There are many devices, which use this approach to monitoring a patient. Some devices use resistance strips which changes current as the strip is elongated, other methods use strain gages and still others use inductance coils which move relative to each other. For example U.S Pat. No. 5,131,399 to Sciarra provides for a respiration transducer with a pair of coiled wires wound side by side to provide mutual inductance.
The mutual inductance signals can be measured easier if the relative movement between the coiled wires is greater. Therefore a device is required which has a close proximity between the coils for high mutual inductance and yet provides for a. larger relative movement when the patient breathes.
A problem in the prior art is that the belts attached around the patient frequently slip or shift changing the diameter of the belt and thereby changing the signals received from the belt. A belt shift to a smaller diameter portion of the body may result in a signal with reduced amplitude, which may be confused with an indication that the patient's breathing is shallower.